Temperature and the larval ecology of the crown-of-thorns starfish, Acanthaster planci, in northern Australia

Adult crown-of-thorns starfish were collected off Gove Peninsula near Nhulunbuy in the Northern Territory on 18 December 1991 and at Davies Reef in the central Great Barrier Reef on the 19th December 1991. The starfish were maintained at the Australian Institute of Marine Science laboratory in running seawater until experiments began in January 1992. Gamete shedding was induced by 1-methyl adenine injection and zygotes were transferred to rearing dishes at experimental temperatures within 5 minutes of fertilization at 24°C.Offspring of adults from both locations were reared in the laboratory at 21°C, 22.3°C, 24°C, 27°C and 31°C to examine geographic differences in development temperature tolerances. The development stage reached by embryos at each temperature was recorded. An experiment was also conducted to investigate a possible parental acclimatization effect in Acanthaster planci. Adults from Davies Reef were separated into two groups. One group was held for 18 days at 31°C and the other at 25°C for 21 days. Tolerance of the offspring of both groups to different temperatures was examined by observing the development of embryos at 18°C, 21°C, 22.3°C, 27°C and 31°C. Cleavage rates were also monitored for both experiments. Four replicate cultures in 20 ml scintillation vials (mean number of embryos per vial = 99) were fixed after 146 min. Embryos in each culture were counted and scored for cleavages completed. Possible parental effects due to seasonal change in water temperature in animals studied immediately after collection were examined. Starfish were collected at Davies Reef in October 1992 when the temperature at collection site was 25.5°C and in November when the temperature at the collection site was 27°C. The development of offspring of the two groups was observed at 21°C, 27°C and 32°C.Because of its importance to dispersal, the temperature tolerance of newly hatched, swimming early gastrulae was assessed. Early gastrulae from cultures of Gove embryos were transferred from 31°C to 31°C (control) for 2 days, 31° to 21°C for 5 days and 31°C to 18°C for 17 days. In December 1992, newly hatched gastrulae from starfish collected at Davies Reef were also transferred from 27°C to 27°C (control) and 27°C to 18.5°C. Davies Reef gastrulae were also transferred from 27°C to 15°C, held there for 6 days and then returned to 27°C. In all cases the development stage reached was recorded and reported as percent bipinnaria development.